Asteroid may have more water than Earth

On Earth, fresh water makes up only a thin layer just a few miles deep in some places, less in others. The water layer proposed for Ceres, while smaller in circumference, is many miles thicker.

The total volume of water on Earth is about 1.4 billion cubic kilometers, around 41 million of which is fresh water. If Ceres' mantle accounts for 25 percent of the asteroid's mass, that would translate to an upper limit of 200 million cubic kilometers of water, Parker said.

Since all the nine "regular" planets have differentiated interiors, this new view of Ceres has some astronomers calling Ceres a "mini-planet," adding fuel to an ongoing debate over exactly what qualifies as a planet.

Embroyonic world
Other researchers recently announced the discovery of 2003 UB313, a round object in our solar system 1-1/2 times larger than Pluto and about three times further away from the Sun. But even an object of this size – at 2,100 miles in diameter roughly four times the size of Ceres – doesn't receive universal endorsement as being a planet.

One astronomer, Brian Marsden, who runs the Minor Planet Center where data on small bodies is collected, says that if Pluto is considered a planet, then any other round worlds should also be considered planets. Under this definition, which some other astronomers subscribe to, Ceres 2003 UB313 and a handful of other large objects would be named planets. The alternative, Marsden and others say, is to stop calling Pluto a planet. 

Another explanation is that Ceres is a sort of 'baby' planet — an underdeveloped version of Earth and other rocky planets. Looked at this way, Ceres appears as other fledgling planets might have looked more than 4 billion years ago.

The leading theory for planet formation holds that small rocks collided, stuck and gradually grew. Depending on location and orbit, a developing world may or may not have encountered enough raw material to become as large as the four traditional rocky planets.

"Ceres is an embryonic planet," said observation team member Lucy McFadden of the Department of Astronomy at the University of Maryland. "Gravitational perturbations from Jupiter billions of years ago prevented Ceres from accreting more material to become a full-fledged planet."

In 2015 scientists will get a close up look at Ceres when the NASA Dawn mission orbits the asteroid. A closer look should provide more clues about the asteroid's composition.

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